A can of worms. I find the JJ KT88s to last forever. The ones in my original amps (2009) are still fine. I run them in class A at 85% dissipation U/L mode. I do the same with KT120s in the bigger amps. None have ever failed. YMMV as always. There was a bad batch of TS KT120s about 6 years back. They would red plate at less than 50% dissipation. Since then I have not heard of any problems from folks using them. Now I use different settings in my main amps (don't need the power)I personally like the KT120s in U/L class A set at 62ma each and 450 B+ with an 8K load. It is really a sweet spot and produces the best sound of all the tube combinations I tried. A fairly close second is running them at 135ma and 500 B+ into a 4K load. Low level output is nearly in the distortion range of preamps. For an amp that uses essentially no NFB (2.5db above audio at 35KHZ for stability) a 0.1% distortion at 1watt makes for a really special sound. I have never tried the 6550 though. Everything from KT77 to EL34. Several brands all do seem to have a sonic signatures that are (IMO) likely hidden in most amps by the use of fairly large amounts of NFB. All Oddwatts (3 watt to 45 per ch) will run fine without any. I just wanted to avoid anything that might excite a 70KHZ resonance that is common to the configuration and selected components.